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Yes, lvalue accessors are pretty useless in the real world. That's because you don't get a chance to intercept the value that's being assigned to. You can't (say) step in and clone something before it's assigned.

Well to be honest, I am not sure what lead you to think that would work in the first place;)

I will admit that $thing->count($thing->count + 1) is ugly, but lvalue accessors are really problematic in a lot of ways.

It has been suggested before that certain types could have additional accessors, so that this $thing->count($thing->count + 1) could become $thing->count_increment() or some such. But of course this is not always desired, so probably wouldnt be a good thing to force on all

I must say I don't feel much for you choice of names for the parameters, most of all, for the "is". It's too vague. This reads just as natural as yours, doesn't it?

has 'x' => (is => 'Int');

And "isa" sounds more like a tradition, than as a good name.

What names would I propose? I don't think I have found really good names, but I have a few suggestions. For "is", I'd call it "access". After all, you're trying to control the access rights to the attribute. As for the other, I'm thinking of something lik

Well, is => 'rw' and is => 'ro' are borrowed directly from Perl 6. And 'isa' has a very well known meaning in the Perl community, so I would rather stick to that then try to create my own. As for dropping the name for the type, this would force an ordering on the options list, which would make things trickier.

The changes you suggest could easily be done using Moose::Policy as well, see Moose::Policy::FollowPBP [cpan.org] for an example. Basically you just preprocess the %options passed to has and turn them i